DC—DC converters are used to convert an input DC voltage into a different output DC voltage having a larger or smaller magnitude, possibly with the opposite polarity or with isolation of the input and the output ground references. Contemporary DC—DC converters should provide a high power density and a high efficiency at a minimum price and high efficiency. The magnetic components of a typical DC—DC converter are often expensive, usually the physically largest, and often very significantly affect the efficiency and reliability of the converter.
DC—DC converters accept input energy from a voltage source at an input voltage and current and provide converted output energy at an output voltage and current, usually to a capacitively filtered output that functions as a voltage sink. When isolation is used, the input voltage is switched on and off at a high frequency and provided to a transformer. The transformer provides both input/output isolation and any required voltage level conversion. However, because the input voltage is typically switched at a high frequency the output voltage and current from the transformer cannot be directly provided to a load in a regulated manner. An inductor is typically required in the energy conversion to act as a current filter. Thus in a typical Buck mode forward converter, two magnetic components are required, an averaging inductor and a transformer.
Recent advances in the DC—DC converter art have included integrating the transformer and inductor functions onto a common magnetic structure or assembly. This is known in the art as integrated magnetics. Advantageously, integrated magnetics reduces the number of components in the DC—DC converter and simplifies the construction of the DC—DC converters thereby lowering the cost and size of the converter and potentially increasing the reliability of the converter.
However, often the integrated magnetics of the prior art do not have an inductor winding because the windings of the transformer, particularly the secondary winding of the transformer, provides the necessary inductance during the magnetic energy discharge operating stage. Thus, the secondary windings of the transformer must satisfy the input/output isolation requirements, the voltage level conversion requirements, and the filter requirements.
The design of a transformer used in a DC—DC application includes a variety of trade-off analyses. These can include the trade-off between copper loss, ferrite loss, and total size. The design of an inductor used in a DC—DC converter includes balancing the number of turns in the inductor and the gap length affects both the inductance and the overall size of the inductor. In general, there is a minimum inductance necessary to provide the required output regulated output current. Often times the minimum required inductance requires a number of turns or a gap length that is incompatible with the size and electrical requirements of the transformer that is to be used. As such, neither the inductor nor the transformer is optimally designed and the performance of the DC—DC converter suffers.
In designing the transformer where the secondary winding also functions as the inductor winding, the number of turns of the secondary winding that are needed to provide the necessary inductance affects the number of turns of the primary winding, the core size, and the output current characteristics. This coupling of the design output inductor and the transformer prevents either from being designed to operate in an optimal manner.
Although some of the prior art integrated magnetics did include a separate inductor along with the primary and secondary windings, in these prior art configurations each winding has an independent output. Thus in these prior art systems, there would be four outputs from the integrated magnetics, two from the secondary winding and two from the inductor. The addition of another output requires additional components to provide a full-wave converter configuration. In addition, in these prior art configurations output current typically continuously flowed through the inductor increasing power losses and decreasing efficiency.
It would therefore be desirable to have a DC—DC converter using integrated magnetics in which the transformer and the inductor functions are decoupled. Additionally, these prior art configurations could only be used for push-pull systems and were not suitable or compatible with full wave voltage inverters.